screen

The ISO++ double screen system from Bom Group is unique in the horticultural industry. It is an innovative double screen system which lets more light through and saves more energy.

The two parts of this patented double screen can be operated individually despite being just 6 cm apart. The two fabrics are attached to the upper beam of the greenhouse trellis girder, keeping the lower beam free from attachments so it can be used for other mechanical installations such as crop wires or a third screen.

Minimum light loss

Because the two screens are so close to each other, when they are both closed a cushion of stationary air forms between the two screens, saving energy and producing an insulating effect. The proximity of the two screens also helps minimise light loss.www.bomgroup.nl
Stand number: 08.108

Next Generation Growing has played a major role in the choices Dutch orchid grower Rob Olsthoorn of OK Plant is making for the new greenhouse he is building in Naaldwijk. “We want to grow in a closed environment as far as possible. The three screens in the greenhouse are helping us achieve that.”

The bottom screen is a transparent, energy-saving fabric. The middle screen is a shading fabric with an open structure which shades 55% of the light. For the top screen, Olsthoorn opted for a light reduction fabric. This screen keeps 99% of the assimilation lighting inside the greenhouse and he can also use it as a blackout when necessary.
The glass is highly diffuse. “This way, I get good light distribution with my triple screen,” the Westland-based orchid grower says, walking through his new greenhouse. “The middle screen shades 50% of the sunlight. That’s plenty. I’m not a fan of screening too heavily. It’s better to have too much light and to have to add in the other two screens than to have too little and not be able to adjust it. It’s important for us to be in complete control so we can create the growing conditions that are best for us. Everything revolves around quality.”

As closed as possible

Olsthoorn grows Phalaenopsis in 9 cm pots on 6 hectares, the last 2.5 hectares of which are currently being built. He has another 2 hectares on which he grows seasonal products like cyclamen, campanula and Primula obconica. “We are targeting the higher segment, such as wholesalers and garden centres. We don’t supply the mass market but specialise in plants with solid added value. And that needs us to be completely focused on quality. Some customers are so strict that they raise the alarm as soon as they see a bad leaf or a mark.”
Next Generation Growing plays a major role in improving quality in Olsthoorn’s greenhouse. “The climate, RH and energy consumption must be as stable as possible so that we can grow in as closed an environment as possible.”
So the nursery opted for a triple screen system which was built into the structure of the greenhouse. You won’t find any end strips made of fabric there. In this part of the nursery, the greenhouse builders Technokas built white steel plates into the greenhouse structure at each end of the screen installations. As a result, the greenhouse is completely energy-efficient and light-proof for its entire service life.

Maintenance-free

“Sitting down with the right partners in the preliminary phase makes a big difference. This system is actually a screen system 2.0,” says Jeroen de Jonge of Peter Dekker Installaties (PDI). “It saves us a lot of work. Our installers used to only be able to start making the fixed strips once the construction stage was finished. They had to squeeze in around heaters, water pipes and lamps to secure the strips for the three screens to the greenhouse separately. With this new system, the greenhouse builder hangs up the metalwork as the building work gets under way. Then all we need to do is pull up the screen, secure it – and that’s it. This system is much less prone to breakdowns and it’s maintenance-free.”
The grower is also hugely impressed with the system. “I’m ready for the next 20 years now. It’s an investment but it will easily pay for itself in the long term. The closed system keeps the greenhouse climate much more stable and it’s quite a lot easier to maintain. I no longer have any flaps that keep coming loose when I spray down the greenhouse.”

The triple screen they went for in combination with highly diffuse glass also serves a purpose. “It means we can use a transparent energy screen that lets in maximum light. It allows the sunlight in in a much more evenly distributed way.”

More resilient crop

The plants under the screens warm up more slowly, enabling them to absorb more sunlight and therefore more UV radiation, the orchid grower believes. “That produces a stronger plant and therefore a more resilient crop.” He has seen it in Asia with his own eyes. “They grow in plastic greenhouses there. I took my light meter with me and I discovered that they let in a lot more light than we usually do here. And yet the plants were still a rich green colour.”
To prevent the climate from becoming too humid, he is installing a dehumidifier unit in the new greenhouse which blows dry air in from outside. “This means I won’t have to adjust the screens so much and the temperature inside the greenhouse will stay more stable.”

Double top wires

The triple screen has one downside: the screens are very close together. On a 60 cm high truss there are three axles which can rotate independently. If the fabric blows up there is a risk that it can become trapped in the turning axles. “Of course, three screens are always riskier than a single system,” says De Jonge. “So we have installed systems that mitigate that risk. For example, we fitted an axle guard on the axles, like a kind of emergency brake. If there is a malfunction, the motors stop, preventing any consequential damage. Fitting double sets of top wires to prevent the fabric from being blown up is also a must with a triple screen.”
No matter how well the greenhouse is equipped with screens, diffuse glass and a dehumidification system, vents remain a problem. To solve this, Olsthoorn has opted for one-sided ridge ventilation. “Fortunately we were able to build the greenhouse in such a way that the vents could be fitted on the north-eastern side of the roof. If the sun shines at noon and the vents are open, no direct sunlight enters the greenhouse, so the plants don’t get burned.” He is not bothered about the wind. “Ninety percent of the time it comes from the west.”

Outdoor screen

Not only are they building a new greenhouse, they are also adapting the existing one to improve the quality of the crop. The grower also has a triple screen there: an 88% screen fabric, a 66% fabric and a diffuse, transparent screen. “This plus the combination of float glass and 50% chalk screening isn’t enough to further optimise crop quality,” Olsthoorn explains. So fitters are also installing an outdoor screen above the greenhouse roof. The space between the greenhouse roof and the screen can keep the temperature inside the greenhouse around 6-7ºC cooler than outside.
Olsthoorn opted for a 50% screen fabric with an open structure. “That means I don’t need to use forced cooling so much and I need to use less artificial light in dark weather in the summer. That saves a lot of power,” he says.
This may be difficult to justify financially compared with his chalk screening, he admits. “But it will pay off in the long term. The outdoor screen gives us complete control over the weather conditions. If it only helps us deliver fractionally better quality, we will have achieved what we set out to achieve with the outside screen.”

Summary

Grower Rob Olsthoorn of OK Plant in the Netherlands deliberately opted for a triple screen for his new greenhouse so as to make it as closed an environment as possible. To improve climate equality even further, he integrated the screen into the greenhouse structure, producing a greenhouse that will be completely energy-efficient and light-proof for its entire service life. The existing greenhouse is being fitted with an outdoor screen which will make the weather conditions completely controllable. The aim of all these measures is to improve quality.

A new online app quantifies the effect of radiated heat loss on crop temperature and energy loss from the greenhouse in a simple, user-friendly way. The radiation monitor is a handy tool for growers who want to get a better understanding of aspects such as the use of screens and greenhouse cover materials. More knowledge of physical and phytophysiological processes in the greenhouse and the crop can help the grower produce even better results.

Anyone with a PC can use the radiation monitor. The app was launched recently, and Aat Dijkshoorn, Next Generation Growing (NGG) project manager in the Netherlands, is very happy with the result. “This program makes it easy to calculate the effects of screening on energy consumption and vertical temperature distribution. It helps growers take concrete decisions – such as whether or not to close the screens tonight – and supports the trend towards energy-efficient growing.”

Know your temperatures

Knowing the plant temperature helps the grower grow more efficiently and accurately. As crop adviser Peter Klapwijk recently put it on HortiNext: “When I talk to growers about their climate strategy, I often realise that they still see the greenhouse air temperature as the most important reference variable. Many of them understand the importance of plant temperature but dismiss it because it’s ‘so difficult to measure reliably’. So people don’t tend to pay much attention to it. But this is a misconception because it is essentially the temperature of the plant that determines the crop’s growth rate and how it is steered.”
Measuring plant temperature is by no means easy, Dijkshoorn admits. Temperature is a result of all the energy flows that occur inside and outside the greenhouse. A simple sensor unit or thermal camera will only capture part of all that data. Then there’s the problem that the equipment needs to be incredibly accurate to register the differences, which are often only a matter of decimal places. I’m convinced that the radiation monitor makes that a thing of the past as well. The simulation model gives an excellent picture of cause and effect, which makes the plant temperature much easier to steer accurately.”

Relative humidity and transpiration

It’s a well-known fact that there is a link between screening and temperature, and the use of screens has risen substantially in recent years as NGG gains in popularity. So it’s no surprise that all kinds of initiatives are being launched to attempt to shed more light on this relationship. The radiation monitor does that very well.
The program was devised by Wageningen University & Research in the Netherlands. Researcher/developer Feije de Zwart understands exactly what lay behind this assignment. “The fact is that many growers are still reluctant to use screens intensively and will only close their screens if the difference between the indoor and outdoor temperature is more than 10°, for example. They understand straight away that a screen saves energy but what they often don’t realise is that it can also bring about more homogeneous vertical temperature distribution in the greenhouse. Many growers mainly see screening as a way of increasing relative humidity. And that can be risky. After all, the more humid the air in the greenhouse is, the lower the difference in vapour pressure between the crop and the greenhouse air will be and the less the crop will transpire.”

Screening is good

Time and again, practical experience shows that intensive use of screening is not necessarily detrimental to crop quality and production. The same conclusion was reached in the “Transpiration at the head” study. This study revealed that reducing radiated heat loss by screening substantially increases the temperature at the head of the crop, leading in turn to higher levels of transpiration at the head. Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the head of the crop. Increasing understanding of this phenomenon by explaining the theory, demonstrating measurements in practice and producing a software tool that quantifies the various effects could help raise awareness of the importance of screening even further.
“That’s exactly what the radiation monitor does,” Dijkshoorn points out. “The application produces the numbers to back up what we have been seeing in practice for some time, namely that the use of screens not only helps save energy but also benefits the crop. It tells you exactly when you can expect to save energy. As I said before, with this model at their fingertips, growers can optimise their use of screens even further.”

Input screen

The radiation monitor calculates the energy balance (the sum of the incoming and outgoing energy flows) based on a large number of relevant parameters. De Zwart: “As everyone knows, the most important parameters are the outdoor and greenhouse air conditions, the greenhouse envelope, the number and type of screens, the crop and any lighting used. The physical properties of the greenhouse envelope, screens and lighting then determine exactly what the energy balance will be. When we wrote the program we decided to show these parameters in the extended help document but without making them editable.”
Other properties can be configured by selecting different greenhouse roofs, screens, crops or lighting systems, but not by changing the parameters at user level. “This way we can guarantee that only realistic parameters are used. The model then calculates the temperatures at various crop heights and, where applicable, at projecting parts of the plant, such as the flowers on gerbera, for example. The app also displays the energy consumption and light intensity at crop height. That is relevant when lighting or transparent screens are used during the day.”

Comparing scenarios

According to Dijkshoorn, the application really comes into its own when different scenarios are compared. “Then you can select a scenario with the screens open and compare it with a scenario with one or two screens closed, for example. But it is important to choose realistic values for the greenhouse climate, especially as the RH in the greenhouse can change,” he says. Kas als Energiebron, one of the initiators of the project, envisages even more uses for the model and would like to see it extended to include more selection options. “For example, at the moment you can only choose between a closed screen or an open one,” Dijkshoorn explains. “But in practice some growers work with screens closed 80% of the way. How does leaving these gaps impact on the temperature? These are aspects that the model can fine tune even further.”
The user-friendly radiation monitor app is already available to growers. It uses a minimal number of input fields: just enough to produce useful calculations of the effect of screening while offering the user plenty of scope for selecting starting points they can recognise. Detailed instructions for use are available on the website and an instruction video is currently being produced. The people behind the app also hope to encourage growers to use the app in workshops, information sessions and through the Next Generation Growing course.

Summary

The radiation monitor displays the realistic effects of greenhouse roofs, screens, lighting and other user settings on temperature distribution in the crop and light intensity at the head of the crop. The monitor also shows temperatures on the surface of the screen and the greenhouse roof. All this gives the grower a better understanding of the effect of screening. The program is an internet application that can be operated on the PC.

Chrysanthemum growers Arcadia and Van Uffelen are definitively going to use 'The Next Generation Growing' at their nurseries in De Lier and Maasland. Technokas was asked to equip the companies with the air conditioning systems needed.

Van Uffelen’s greenhouse was designed and built by Technokas two years ago. The chrysanthemum nursery was also prepared to some extent for the installation of the climate system required by The Next Generation Grwoing. The Arcadia installation is part of a new project involving replacement and expansion at an existing site. The installation will suck dry outside air via the gable ends, possibly mix this with greenhouse air, and if necessary heat this locally until it reaches the indoor temperature. It will then be distributed among the cultivation departments with transparent hoses.

Portals in gable ends

Air handling units (AHUs) will be installed in the purpose-made portals in the gable ends. The greenhouses will be provided with manifolds and mix groups for the heating elements, where these do not already exist. In addition, the installations for sprinkling and lighting are designed in such a way that the air distribution tubes can easily be hung over the crops, and do not negatively affect the reach of the treated air from the holes in the hose. These greenhouses are also provided with a second energy screen.

Long-term testing

Arcadia and Van Uffelen decided to use this climate system as a result of the good results achieved with them in one of Arcadia’s other sites. Wageningen UR, Kas als Energiebron, DLV, Deliflor and other chrysanthemum growers have been testing this system for a long time. During prolonged tests with other crops, positive results in terms of controllability of climate requirements and energy efficiency have also been achieved using Technokas’ climate system.

Energy savings and a better climate

Growers expect the Technokas installation to help further improve the regulation of temperature distribution and relative humidity of the climate in a cultivation department. In addition to a more uniform climate, growers expect that this, in combination with the energy screens, will result in 15 to 30 percent savings in energy consumption. The technology is also expected to lead to even better quality chrysanthemums.

Energy efficient production according to the principles of Next Generation Growing, without any additional investment, is the aim of pepper trial being carried out at the Delphy Improvement Centre (IC), Bleiswijk, the Netherlands. Armed with two energy screens and fans the trial participants want to save 30% on energy and still achieve good fruit quality.

The main barriers raised by pepper growers to grow as energy efficiently as possible in practice are doubts about the impact on crop health and fruit quality. This therefore was the reason for running two climate trials this year with peppers, one at the Improvement Centre and the other at neighbouring Wageningen UR Greenhouse Horticulture.
The one at the IC is being carried out in an area covered with a standard greenhouse roof and two energy screens. The other trial is taking place simultaneously in a VenlowEnergy greenhouse with double glazing. The red variety Maranello was planted in both greenhouses on 7 December 2015. A Supervisory Committee, which includes four pepper growers, is following the trials closely.

Energy savings

The trial at the IC uses two transparent energy screens, namely Luxous 1547 D FR and Luxous 1347 H2no FR. The H2no property ensures that when the screen is used during the day it also allows a lot of light to penetrate even when it is wet due to condensation. In addition the area is equipped with horizontal and vertical fans and is sparsely heated. Although the energy consumption on commercial nurseries is usually about 30 m3 per m2 the trial participants are aiming for 20 m3 per m2. That is a saving of more than 30%.
Assessing the balance half way through the trial it would seem that the goal is achievable. Energy consumption is even slightly lower. Maximum use of the energy screens and omission of the minimum pipe rail therefore have a huge impact.

Screening based on radiation

The principles of Next Generation Growing (NGG) were applied during the trial. The use of the energy screens is the dominating factor. The upper screen opens when the radiation is 100 watt per m2. The second screen opens at the moment that the temperature above the screen differs by four degrees from the desired heating temperature. This small difference should prevent a cold dump.
There were moments this spring that the lower screen was still closed when the radiation was 300 to 500 W/m2. The intensive use of the screens has, from the start of the cultivation to early April, led to 14% light loss. That was difficult for the growers to get used to as they prefer to allow in as much light as possible.
“But the crop was growing to our liking,” says Rick van der Burg, crop manager at the IC. “We noticed that the room temperature was quickly a degree higher than what is usual in practice," adds Arie de Gelder, researcher at Wageningen University & Research.

Drain off moisture

At the same time the screens play a major role in the removal of moisture. At the moment that the RH becomes too high, cool dry air is supplied via the vents above the screens. The moisture is then removed to the outside via transport through the screens. Therefore the usual method of making a gap in the screen is not used,
The fans ensure a uniform temperature and moisture distribution in the greenhouse. At the start of the cultivation this was achieved by just using the horizontal fans. As the crop becomes taller the vertical fans are used too.
The trial participants are not completely satisfied with the air currents and thus the temperature distribution that occurs in the section. Bubble wrap is attached to the walls to rule out influences from outside and from the adjacent much warmer section. Because so little heating is used the temperature differences between the walls has relatively large impact. “We’ve noticed that strong air currents occur,” says De Gelder. That will be different in a practical situation.

Radiation

When Van der Burg made the first assessment in mid April, it revealed that 2,500 hours of screening were with a double screen. That saved a lot of energy especially in March and April.
A net radiation sensor was hung in the top of the greenhouse. This shows how much radiation enters the greenhouse and how much radiation is emitted from the crop. The double screen in the night leads to an important reduction in the radiation emitted.

Screen out the light

As the radiation increases, the screens will be used as a tool to screen out excessive light. The greenhouse does not have a solar reflective coating. When the radiation is more than 700 W/m2 the upper light diffusing screen closes 80% and the lower screen 40%. They are positioned so that they overlap each other. De Gelder: “In this way we want to keep the humidity as well as the CO2 as much as possible at the right level."
Initially ventilation only happened when the greenhouse temperature was more than 27°C. Since the greenhouse temperature rose rapidly at high radiation it was decided to slightly reduce the temperature. Van der Burg: “We noticed that the fruits then become wet and we have to prevent that.”

24-hour temperature based on radiation

The desired greenhouse temperature is very dependent on the radiation. During dark periods the 24 hour temperature is 18.5ºC. When the light sum is 1,000 joules the 24-hour temperature should be 20.5ºC and at 2,000 joules it should be 22.5ºC. The light sum of the previous day determines the night temperature that follows.
It’s noteworthy that no minimum pipe rail is used. Heating is only used when there is a need for energy. Incidentally, plant temperature is well monitored.

Fruit quality

What is now interesting is how the crop responds to these climate settings. In particular the growers in the Supervisory Commission, who are willing to push to the limits, have been amazed at the crop condition. They didn’t expect the crop to look so good after so much screening and the subsequent loss of light. During the first setting some fruits aborted so the trial participants didn’t have to consider thinning out. The first setting started to develop a little later than in commercial nurseries but the differences weren’t shocking. Harvesting started in week 12 and by week 18 the yield was 6.10 kg/m2.
The second part of the cultivation will be interesting when the radiation rises even higher and the crop develops further. Then the emphasis will be more on the vertical temperature distribution in the greenhouse. Of course the growers and researchers are closely following the quality of the fruit. Everyone is wondering what the final fruit quality will be like and what affect the climate regime has on the total yield.

Summary

A pepper trial with NGG in an existing greenhouse in the Netherlands shows that during the first half year a lot of screening has no adverse effects on the crop or yield. Up to now it has been easily possible to save 30% on energy. The two energy screens limit the radiation during the night so the crop temperature remains higher.

The life span of mechanical drive systems for screens mostly depends on the frequency of use, the amount of load, the technical implementation, the quality of the materials used and maintenance. In a well-laid and maintained screen system it’s the frequency of use and the load that are the two most important factors.

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Assimilation lighting has a big impact on the greenhouse climate and - when the electricity is generated on site - on the efficiency of the energy generator. Optimising the control of the lighting installation leads to a gain in both areas. Two growers and their suppliers explain how they achieved it.

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You’d think that the climate in modern, well insulated greenhouses would be a lot more homogeneous than in days gone by. Nothing could be further from the truth, says climate specialist Bas Knoll, of TNO, the Netherlands. In a project taking several years he is assessing the causes and solutions and is working on a climate model that should offer growers and system developers more understanding.

Greenhouses have by definition a heterogeneous climate, both horizontally and vertically. That also applies to modern, well insulated greenhouses. “The temperature differences increase as the heating becomes more intensive and horizontally this can reach more than 5ºC,” says climate researcher Bas Knoll. “As a consequence the RH can also vary from place to place by 15%. That has implications for the heating system. Actually the differences are the largest in highly insulated, large greenhouses."

Calculated risks

To reduce the internal differences growers need to make very local adjustments. That rarely happens. Firstly because equipment such as air vents and heating groups are not sufficiently geared up for that. And secondly because the majority of nurseries have too few data collection points to accurately measure and follow the climate differences.
“This is the direction we need to go in the long term and most growers recognise that,” according to Knoll. “In addition there’s a sharp rise in Next Generation Growing. Here you can take more calculated risks and thereby get more from the crop and at the same time save energy. The condition is that you have the maximum grip on the climate. That is only possible if the differences in temperature and RH inside a greenhouse or area remain small."

Monitor and model

Initiatives to bring the homogenisation of the greenhouse climate to a higher level have resulted in some progress but the whole picture about what works and what doesn’t is far from clear. This was a reason for the Dutch ‘Greenhouse as Energy Source’ project to invite TNO to make a critical evaluation and to develop a simulation model. Its aim is to give growers and suppliers more grip on climate control and the systems involved in that.
Researcher Knoll: “Many whole and half truths are doing the rounds and everyone is struggling with the question what can we do? To clarify that we have to take measurements over a long period in greenhouses to discover when and to what degree climate differences occur, to make the connections and determine the influencing factors. Because it is very difficult for growers at any given moment to see through the interaction of factors there was an urgent need for a simulation model that offered the desired insight and understanding. In addition, there had to be an overview of available and yet to be developed solutions to be able to solve the most important bottlenecks."
These steps are now in motion according to the report ‘More homogenous climate in greenhouses' that TNO published recently. The last step that still needs to be made is the verification and tightening up of the simulation model, as part of a design platform that has gained the abbreviation SIOM (System integration and Optimization Model). Preparations are in full swing.

Cause of temperature differences

Various causes can be the basis for the increasing horizontal temperature differences. Firstly the heating units installed in well insulated greenhouses often have narrower dimensions so are slower and there is a suggestion of higher temperature gradients in parts of low-value heating networks. Also, during the refurbishment of greenhouses the wall heating often remains unchanged.
The second factor is the wind and window vents (see figure 1). Wind that blows over a greenhouse with (partly) open windows always results - due to local over and under pressure - in uneven natural ventilation. Relatively cold outside air comes into the greenhouse furthest away from the windward side (see supply), while nearby on the sheltered, leeward side, warm greenhouse air is removed (see outlet). In addition, the temperature gradient on the roof is barely taken into account.
An equally underestimated point is, according to Knoll, the accuracy of wind and wind direction meters. Often the meters are too low, so that the measurements are influenced by nearby objects, such as high buildings or chimneys. “And an inaccurate measurement leads to inaccurate control of the air vents," says the researcher.

Cold dump

Thirdly, screens and their use make a small contribution. Notorious is the cold dump that occurs by making a gap in the screen but also when the screen is fully closed often a structural localised cold dump occurs through small gaps in the screen. This forms the motor behind internal airflow and temperature gradients. “In addition there is often an imbalance because horizontal screens work dynamically, while the wall screen is permanently insulating,” adds Knoll.
The list of possible causes is easy to expand when other, often crop- or nursery-specific factors are included. Examples are artificial lighting, which are often switched on and off in groups, variations in crops, limitations of the greenhouse and (other) installations, design flaws and so on. In addition, the change to a new cultivation strategy and the installation of new equipment can disturb the precarious climate balance.

Cures

To effectively reduce climate differences within a greenhouse or an area two things are essential, argues the TNO researcher. First of all, more data collection points are needed to indeed be able to measure those differences. “I know a gerbera grower who intensively installed sensors but otherwise hasn’t invested anything,” says Knoll. “By better following the internal climate differences and eliminating them as much as possible with minimum means, he says he’s been able to save tens of thousands of euros annually.”
The researcher also says it’s advisable not just to control the greenhouse or even a section but to narrow that down further. “For example, consider varying the window openings in small sections and be more focused on the internal air circulation,” he suggests.

Simulation model

There are many aspects and possible interactions therefore that need to be fathomed out. This comes together in a new simulation model that helps to find the right combinations. When such a climate model is in place and has proved reliable, it offers several advantages, says the researcher: "You can use it to develop improvements and innovations in order to optimise necessary systems in terms of capacity, lay-out and energy-efficiency. You can also use the output of the model as input for improved control and management of the climate.”
This climate model is part of the design platform SIOM. This is certainly not intended to reinvent the wheel on every terrain, but to bring the multitude of existing computing and design models made by different parties under one umbrella and to serve as a platform for their integration. It also uses new information structures and decision support technologies.
“The climate model is currently only used for research. It has been tested in various forms and cases and enthusiasm for it is steadily growing,” according to Knoll. “The next step is to involve external parties in more practical exercises. It is still ‘work in progress’. To see the real benefits of the model we now want to collaborate with greenhouse and equipment designers who want to be out in front.”

Summary

A project spanning several years is assessing the causes and possible solutions for climate differences in greenhouses. The results have been taken into account during the development of a design platform, in which many existing design and calculation models have been integrated. This should give both growers as well as builders of greenhouses and equipment more understanding about how to achieve a more homogeneous climate.

Gertjan van der Spek is the first tomato grower without artificial lighting in the Netherlands to have two transparent energy screens and no dehumidification. This was his next step to save energy after having reduced the use of the minimum rail and ventilating above the screen, instead of making gaps in the screen. In this way he hopes to use no more than half cubic metre of gas per kilo tomato.

The idea of using a double energy screen for vegetable production is not new. A growing number of pepper growers are doing this. “Pepper plants grow less quickly so the screens can stay closed for longer. The switch to using two screens is less great for them,” says climate specialist Paul Arkesteijn, of screen manufacturer Svensson.
To show that possibilities exist for tomato growers as well, a demonstration trial was run at the Delphy Improvement Centre in Bleiswijk, the Netherlands, last year. This trial compared an area with two moveable transparent screens with a standard transparent screen with a fixed anti-condensation foil.
The top layer was the ordinary Luxous 1347 FR transparent energy screen; the bottom layer was the 1347 FR H2no with an anti-condensation property. The condensation droplets flow out of the latter screen. According to Arkesteijn both transmit 80% light. When both are closed, they still transmit 64% of the light. The crops in both greenhouses grew well. The trial greenhouse saves an extra 4 m3 gas/m2.

More savings

Gertjan van der Spek, of greenhouse company Solyco, with two locations near Rotterdam, grows Roma tomatoes on 4.3 ha. He is part of a horticultural cluster comprising six companies. They have a joint boiler house which has three energy sources available: waste heat from the ROCA-power station; two CHPs; and a boiler. The latter also serves as a backup if there is a breakdown in the CO2 supply.
“Our cluster wanted to purchase a heat pump to further cool the flue gases from the CHPs. Now the flue gases are around 45 to 50ºC and we want the heat pump to cool them to 23ºC. The two CHPs use 900 m3 gas per hour and the recovered heat corresponds to approximately 130 m3 of gas. To be eligible for a subsidy scheme run by the Netherlands Enterprise Agency the cluster had to save in total 15 per cent on energy. That meant each grower had to make extra effort,” explains the tomato grower.
This took place at the time that Van der Spek became one of two growers to join the supervisory commission for the trial at the Improvement Centre. Hence the reason for him being able to pay extra attention to the trial with the two transparent screens.

Second energy screen

During the spring the trial appeared to be running so well that Van der Spek dared in August last year to install a second energy screen under his existing screen. The fact that his first screen of eight years old was becoming more porous and therefore was in need of replacement also played a role. He had to invest in a second wiring system for the second screen but that wasn’t a problem, even though this option was not taken into account during the original construction. “Screen installer Alweco came up with the solution of attaching the second screen half way across the trellis. For safety we ensure that the cloths are not moved at the same time because it’s during opening and closing that most of the force is placed on the walls.”
The grower chose the light-transmitting Luxous 1347 FR, without anti-condensation properties. “The anti-condensation property is not necessary for us since we mostly screen at night.”

1,500 double screening hours

The planting date was 1 December and up until the end of April he regularly used two energy screens. The grower always closed the new screen first. Up to week 20 he had accumulated 2,200 hours of screening. He used the old screen for 1,500 hours.
Compared to last year he used 0.5 m3 more gas during this period for a slightly longer production. “In terms of energy consumption, we are about equal to last year, but then it was very mild compared with this year.” When he compares his energy consumption with that of colleagues he uses about 2 m3 less than growers with a fixed AC-foil and 5 m3 of gas less than growers with a single screen.
During the first six weeks he hardly made any savings compared with growers with the fixed AC-foil. “During this period the main advantage of a moveable screen is flexibility. I already had some production advantages because with a moveable screen you have fewer problems with moisture. We never wanted to have a fixed foil. I always had the idea that it only became cold when the foil was removed.”
The trial in Bleiswijk also used the double screens intensively during the autumn months, from October. That achieved extra savings of 1 to 2 m3. “During the last few weeks of the cultivation period the temperature needs to be high enough to allow the tomatoes to ripen properly.”

The New Thinking

In 2014, when the winter was mild, the grower used 27 m3 gas per m2 and yield was 63 kg. In a ‘normal’ year that is around 32 m3. “We now want to make these savings with the double screen cloth and we hope again to have a yield of 63 kg,” says the grower.
Van der Spek has already made huge steps on his nursery over the years. When he started in 1992 he used 72m3 gas. After building a new greenhouse in 2000 consumption dropped to 50 m3 gas. In 2005 that was reduced further to 40 after the installation of his first screen. The step to 32 m3 happened mainly thanks to new insights into production. Yield increased from 50 kg in 1992 to around 63 kg of tomatoes today. “Production rose to more than 65.5 kg thanks to grafting but we’ve scarified some of that as we save energy.”
The result is impressive: from 1.5 m3 gas previously to now 0.5 m3 gas per kg tomato. Arkesteijn attributes the new cultivation insights to The New Thinking, a derivative of the Next Generation Growing. “They used to make a gap in the screen to release moisture but consequently suffered a cold dump. That resulted in horizontal temperature differences. The climate is controlled based on the coldest spots. Now growers keep their screens closed for longer and ventilate the moisture away through the cloth screen. The big advantage is the uniform climate. “That is not only beneficial for saving energy, but also for product quality.”

Use minimum rail sparingly

The tomato grower says he has applied this strategy for two years. Firstly he only ventilated on the sheltered side. Now, when using two energy screens at once, he ventilates on both sides to achieve good air movement above the screen, so that the air is drawn through the screen cloth. And instead of opening the windows just a little he now dares to open them wide. Another adjustment is that the grower uses the minimum pipe very sparingly. “We have dropped from 50ºC to 40ºC and now to 30ºC.”
Arkesteijn: “Previously we believed you had to heat the crop for it to continue to transpire. What matters now is that the crop transpires sufficiently during the day. The energy screen closes at 80 to 100 watt radiation. The crop is activated the next day by the sun that shines through the particularly transparent screen."

Summary

Tomato grower Gertjan van der Spek is the first tomato grower in the Netherlands who doesn’t use artificial light to have two transparent energy screens without any dehumidification system. After a series of energy saving measures, such as the first screen, ventilating above the screen cloth and seldom use of the minimum rail, he took the decision to install a second wiring system and a moveable screen. He hopes this will result in gas consumption of 27 m3 and yield of around 63 kg per m2.

In a large greenhouse with supplementary lighting the temperature differences in winter can rise so high it’s at the expense of quality and energy consumption. Berg Roses, of Delfgauw, the Netherlands, has already broken new ground with the Next Generation Growing and is fully committed to improving the climate. After a trial with vertical fans it is now running a trial with horizontal fans.

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